In the production of today's integrated circuits, optical lithography is one of the key techniques. The ongoing miniaturization of integrated circuits or other devices results in a number of problems, which may be encountered during optical lithography. When, in an optical lithographic system, light generated by a light source is incident on a mask, the light will be diffracted. The smaller the dimensions of the structures on this mask, the more the light will spread. Hence, the smaller the dimensions of the structures on the mask, the less of this spread-out light will be collected by an objective lens so as to be focused onto a resist layer. As a result, the image of the mask structure formed onto the resist layer will be of a low quality. A well known solution to cope with the light spreading and consequently to obtain sufficient quality of the mask image is the use of systems having a high numerical aperture (NA). Typically, immersion fluids are used to deal with the corresponding incidence of light having a high angle of incidence onto the wafer.
Light, which propagated through the resist, is typically partially reflected back into the resist by the substrate on which this resist has been deposited. The latter may lead to multiple interference effects, resulting in a lowered quality of the obtained print. Multiple interference effects result in a variation of intensity with resist depth, causing a variation of the development rate with resist depth. As a result, the resist sidewalls have a scalloped profile or so called ‘standing waves.’ This standing wave problem will cause pattern collapse of lines in defocus, strongly pronounced standing waves at the bottom of the resist line, or incomplete development of lines or contacts holes, especially in defocus.
The multiple interference effects in resist will result in a variation of total absorbed energy with resist thickness, hence in a variation of the critical dimension (CD) with resist thickness. The latter is known as the ‘swing effect,’ which will cause CD non-uniformity if patterns have to be made on substrates with topography. These multiple interference effects in the resist, and hence the total amount of energy absorbed, will also depend on the layers underneath the resist and variations thereof in thickness and/or composition. For example, the interference effects will be different if a contact hole is to be printed in a resist layer on top of an oxide or on top of a nitride layer, as both dielectric layers have different optical properties, such as transparency.
In lithography applications, typically bottom anti-reflective coatings (BARC) or bottom anti-reflective layers (BARL) are used underneath the resist to decrease the effects of multiple interference due to reflection by the substrate. As used herein, the term bottom anti-reflective coating will be used to refer to both BARC and BARL, as commonly used by persons skilled in the art.
Using bottom anti-reflective coatings, the reduction in substrate reflectivity can take place in two ways: by absorption of light in the BARC or by destructive interference of light rays at the bottom of the resist. The latter is illustrated in FIG. 1, showing a part 100 of a lithographic process step. A device 102 is covered by a resist layer 104, and a BARC 106 is sandwiched between the device 102 and the resist layer 104. The light rays 108 show the situation whereby light is absorbed in the BARC, which is only possible if the BARC is sufficiently thick. Unfortunately, the etching of a thick BARC layer with the resist as a mask is often a problem due to excessive resist erosion.
The light rays 110 show the situation whereby reflection is reduced by destructive interference, which is only possible if the BARC thickness is everywhere exactly the same, causing the required phase difference between the interfering light rays. The latter even may be obtained on topographical substrates, e.g., using inorganic BARC's. Some BARC's, such as inorganic BARC's, show planarization over topography, causing BARC thickness variations. Hence organic BARC's are typically used combining interference effects and absorption in order to reduce the substrate reflectivity on topographic substrates.
The patent application entitled “Method and system for BARC optimization for high numerical aperture applications” by IMEC vzw, co-pending herewith, describes the optimization of a single BARC layer depending on the pattern, i.e. pitches, on the mask used. If large ranges of pitches need to be patterned, a single BARC layer does not allow sufficient control of the reflections towards the resist layer for all pitches. Consequently a stack of bottom anti-reflective coatings may be used. Nevertheless, optimizing the BARC stack may be very time consuming. For example, for a dual BARC system, six parameters need to be optimized for all pitches under consideration simultaneously. These six parameters are the real refractive index, extinction coefficient, and thickness of each BARC. The latter is very time-consuming.